In radiological practice, the radiographer frequently needs to locate specific anatomic structures on the radiograph in order to focus his attention on these specific image areas. The position of the image areas will typically depend on the examination type of the radiograph and the suspected disease; hence these locations are examination and diagnosis-specific.
For example, in bone age assessment according to the Tanner-Whitehouse method, 20 bony regions of interest (ROI) need be located on a hand radiograph, and staged against a set of reference stages depicted in an atlas. In European patent application EP A 1 293 925, a method is disclosed to automate the staging and scoring of these skeletal sites. The associated ROI of these sites is extracted and compared against a set of reference pictures and diagrams by presenting a given ROI simultaneously in close vicinity with its reference set. The location process for each of these regions of interest must be performed irrespective of the position of the hand itself in the image. A typical exposure error introducing positional variation is that the hand axis is placed in a rotated fashion with respect to the cassette borders of a cassette conveying a radiation detector such as a photo-stimulable phosphor screen on which the radiation image is recorded. Furthermore, irregular finger spread of fingers adds a second degree of rotational variation in the location of these ROI's. Hence, there is a need to enhance the location and extraction process of ROIs in a medical image.
A further location determination task in the Tanner and Whitehouse scoring method is that anatomic bony features are compared against geometric objects in relationship to anatomy. For example, in the scoring of the scaphoid, the stages G and H are characterized in that the direction of the lunate border of the scaphoid is compared with respect to the midline of the capitate. Therefore, the radiographer must continuously mentally map the capitate midline into the image in order to verify this feature and assign the correct stage to the scaphoid. Hence, there is a need to map this diagnostic line into the actual image, so as to avoid the need of continuous mental relocation.
A second diagnostic activity where objects need be located in an image is the field of geometric measurements. Obviously, in the film-based measurements, no assistance is offered in drawing the geometric objects or entities in the image. In European patent application EP A 1 349 098 a method is disclosed to automate the measurements in digitally acquired medical images by grouping measurement objects and entities into a computerized measurement scheme consisting of a bi-directionally linked external graphical model and an internal informatics model. In a measurement session according to European patent application EP A 1 349 098, a measurement scheme is retrieved from the computer and activated. Measurements are subsequently performed on the displayed image under guidance of the activated measurement scheme.
In this computerized method, a multitude of geometric objects must be mapped in the digital image onto which other measurement objects and finally measurement entities (such as distances and angles) are based. The basic geometric objects are typically key user points, which define other geometric objects onto which measurements are based. For example, two pairs of key points each define a line, and an angle between the resulting line pair, representing the angulation between anatomic structures, is computed. These key user points must be mapped in the image onto their corresponding anatomical landmark positions. In order to facilitate the initial position of these key user points, their position may be computed on the basis of a small number of anchor points, which are also defined in relationship with anatomy and will typically coincide with easily discernible anatomical landmarks.
A further location task when performing measurements in digital images is related to the selection of the image area and the proper resolution of the image for placing the geometrical objects such as measurement points. For large images of which only a downscaled version is displayed, the radiographer will usually prefer to position the key user points in the original image in order to use the full resolution for better positional accuracy of the key user points. Therefore, the image area around each sequential key user point must be displayed in its original resolution, which requires that the user first select the image area to be magnified. This is typically done by methods of the prior art by an additional mouse click in the image in the vicinity of the desired key user point, upon which the image is displayed at full resolution, or by panning in a second image window displaying the image at full resolution until the region around the desired key user point is viewed. However, the drawback of this manner of operation is that when the number of key user points is large, which is typically the case for complex measurement schemes, a large number of mouse manipulations is needed, hence this method is slow, fatiguing and error prone.
A third area in radiological operation that also requires a geometric location is that of quality assurance and quality control by means of an analyzing phantom. For example, in mammographic system quality control, a phantom with a specific spatial arrangement of geometric structures is used. These structures such as disk and bars serve to determine the image contrast and compare it against perceptibility thresholds. Geometric accuracy may be checked by measuring the structures' spatial locations in the image and compare them with their model counterpart. To automate the analysis of the imaged phantom, a CAD model may be used to reference the geometric structures against a coordinate reference system. This reference system may be established by means of small number of control points of known location. For these control points are part of the phantom, they are imaged simultaneously with the analyzing objects. By locating their position in the image, a correspondence between model and actual image can be established. After establishing the geometric transformation and performing the mapping of the model structures, as will be detailed in the sequel, the initial position of all referenced objects is easily found back in the image, and subsequent quality parameters are computed.
In EP 1 349 098 a method has been described for performing geometric measurements on digital radiographic images using graphical templates.
A digital image representation of a radiological image displayed. A measurement template comprising the set of measurement points or areas is displayed adjacent to the digital image. The measurement points or objects are mapped onto the displayed image by manually indicating locations of points in the displayed image corresponding with the points or objects of the template, and the co-ordinates of the resultant mapped points are stored and used for performing measurements on the displayed image. The described method is performed manually.